Added mixins to style with different invalid colors

[gerjanvangeest]

I wanted to use the paper-input but not style the label, underline and add-on in the same color when invalid. In the current situation all three elements where all styled by the variable:
--paper-input-container-invalid-color

With these three new mixins the label, underline and add-on can be styled individually.

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[gerjanvangeest]

I signed it!

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CLAs look good, thanks!

[notwaldorf]

This is pretty against the material design spec, and it would produce a pretty mismatched input, so I'm not sure how I feel about this :(